If this helps at all, I'm running a more sandbox D&D 3.5 campaign with my college buds. In the days between sessions, I usually collect my notes, organize them, and look through stat-blocks and character sheets I've made. I usually mass-produce character sheets and stat-blocks for different enemies or encounters because I feel the more content I can offer to my players, the more depth the world will have. What's a sandbox world without action taking place outside of the party's actions? Sure, they won't work to stop the big-bad. That doesn't mean the big-bad will halt his plans and progress until the party resumes its Main Quest.